Well, it will make mapping the Saturn-facing hemispheres quicker. The trailing hemispheres don't have the benefit of Saturn-shine.

Decepticon

Oct 1 2005, 08:09 PM

Doh! Should of known that.

BruceMoomaw

Oct 1 2005, 08:58 PM

Slight slip-up by Ted: it's the "anti-planet" hemisphere of a moon -- not its trailing hemisphere -- that's naturally deprived of planetshine. (Actually, it would be simpler to use the same terminology for other planets' moons that we use for our own: "nearside" and "farside".)

tedstryk

Oct 3 2005, 12:29 AM

QUOTE (BruceMoomaw @ Oct 1 2005, 08:58 PM)

Slight slip-up by Ted: it's the "anti-planet" hemisphere of a moon -- not its trailing hemisphere -- that's naturally deprived of planetshine. (Actually, it would be simpler to use the same terminology for other planets' moons that we use for our own: "nearside" and "farside".)

You are right. Leading/Trailing refers to orbital direction. Thanx for catching that one.

Bjorn Jonsson

Oct 4 2005, 08:19 PM

I'm attaching a 6 image montage showing Dione at various times surrounding closest approach next week. This flyby is similar to the recent Tethys flyby in the sense that the phase angle prior to closest approach is very low.

Only 90 minutes after closest approach, Cassini will also make one of its closest nontargeted flybys of any moon -- within 9750 km of Tethys' little Lagrange moon Telesto, which will indeed be imaged. (I wonder if this will be close enough to determine Telesto's mass, allowing us to determine whether the Lagrange moons are rubble piles like the inner moons and Hyperion.)

By the way, http://www.aas.org/publications/baas/v37n3/dps2005/603.htm reports that one odd trait of Dione discovered by Voyager -- the fact that there seems to be a concentration of magnetospheric ions near its orbit -- may be due not to outgassing by Dione itself, but rather to some of the gas generated by Enceladus, which is not greatly ionized by radiation until it drifts into the outer parts of Saturn's magnetosphere. Still no word, however, on whether Cassini discovered any indications of outgassing from Tethys during its close flyby.

While Cassini will make a fairly close non-targeted approach to Enceladus, from the recently released Dione flyby description I figure there aren't any Enceladus observations programmed. Instead, the spacecraft will be engaged in a rather lengthy data downlink session at the time of C/A to Enceladus.Too bad, but you can't have unlimited data storage onboard, can you?

QUOTE (BruceMoomaw @ Oct 8 2005, 09:29 AM)

Only 90 minutes after closest approach, Cassini will also make one of its closest nontargeted flybys of any moon -- within 9750 km of Tethys' little Lagrange moon Telesto, which will indeed be imaged. (I wonder if this will be close enough to determine Telesto's mass, allowing us to determine whether the Lagrange moons are rubble piles like the inner moons and Hyperion.)

That would be great, but I wouldn't hold my breath. The flyby guide says there's a couple of radio science observations, prior to Dione C/A and after BOTH Dione and Telesto C/A. Personally, I think Telesto's mass is far too small compared to Dione and the approach distance of some 10000 km is too much to pick out Telesto's mass from the whole measurement. Perhaps if they included a short radio science observation between Dione and Telesto C/A, but even then, who knows...

Decepticon

Oct 8 2005, 07:23 PM

I've always wondered how many Gigs of data SSR can hold?

ugordan

Oct 8 2005, 08:26 PM

QUOTE (Decepticon @ Oct 8 2005, 09:23 PM)

I've always wondered how many Gigs of data SSR can hold?

Cassini has 2 SSR recorders, each with 4 gigabits of storage. That's about 477 megabytes per SSR (or alternatively 512 megabytes if gigabit means 1024*1024*1024 bits, but I think the figure for bits is 1000*1000*1000).An uncompressed ISS full-frame 8-bit image is exactly 1 megabyte, the compression that truncates lines on the right guarantees this to be <=512 kilobytes so you can quickly determine how many images you could store in case all memory is available just for images (which it's not )...

For comparison, I seem to have read that Voyagers' tape recorders could also hold something like 512 megabytes, though I could be wrong on this.

pioneer

Oct 8 2005, 08:49 PM

QUOTE

For comparison, I seem to have read that Voyagers' tape recorders could also hold something like 512 megabytes, though I could be wrong on this

I don't think so. The Galileo tape recorder could not hold more than 150 megs, so I doubt the earlier Voyager could hold more.

More like 512 megabits

ugordan

Oct 8 2005, 08:55 PM

QUOTE (pioneer @ Oct 8 2005, 10:49 PM)

I don't think so. The Galileo tape recorder could not hold more than 150 megs, so I doubt the earlier Voyager could hold more.

I remember reading somewhere that it could hold thirty-something images.

BruceMoomaw

Oct 9 2005, 09:53 PM

More like 100 images per recorder (or a little more; I'd have to recheck, but it's definitely over 100 each).

BruceMoomaw

Oct 9 2005, 09:53 PM

Oh, excuse me; I thought you were referring to CASSINI'S recorders. Voyager's, I don't know about.

tedstryk

Oct 10 2005, 01:12 PM

Does anyone know if the NT of Enceladus will be used? If it covers any "new" territory, it seems like a great opportunity. Also, repeat coverage for Enceladus is much more important than for a moon like Iapetus, since change detection is a priority.

ugordan

Oct 10 2005, 01:45 PM

QUOTE (tedstryk @ Oct 10 2005, 03:12 PM)

Does anyone know if the NT of Enceladus will be used? If it covers any "new" territory, it seems like a great opportunity. Also, repeat coverage for Enceladus is much more important than for a moon like Iapetus, since change detection is a priority.

Earlier in the thread I said:"While Cassini will make a fairly close non-targeted approach to Enceladus, from the recently released Dione flyby description I figure there aren't any Enceladus observations programmed. Instead, the spacecraft will be engaged in a rather lengthy data downlink session at the time of C/A to Enceladus."

From 42 000 km, 74 deg phase, there is an opportunity for some pretty good ISS NA coverage, perhaps a 4x4 mosaic, but as I said, I don't believe any actual observations are planned...

ugordan

Oct 10 2005, 01:54 PM

QUOTE (BruceMoomaw @ Oct 9 2005, 11:53 PM)

Oh, excuse me; I thought you were referring to CASSINI'S recorders. Voyager's, I don't know about.

Well, if the figure for the 500 megabit (around 64 megabytes) Voyager recorder is true, IIRC the resolution of the Voyager ISS cameras is about 800x800 so that's about 640 kb per image. That would indeed turn out to be around 100 images max -- the advantage that Voyagers had over Cassini is that they could downlink new images in real time (at least during Jupiter and Saturn encounters) so the storage limit probably wasn't that big of an issue as is with Cassini.

tedstryk

Oct 10 2005, 03:00 PM

Data Storage Subsystem ---------------------- The Digital Tape Recorder (DTR) was used to store data when real-time communications with Earth were either not possible or not scheduled. The DTR recorded data on eight tracks; rates were 115.2 kilobits per second (record only), 21.6 kilobits per second (playback only), and 7.2 kilobits per second (record and playback). Capacity of each track was 12 images or equivalent.

My original figure was based on the Uranus encounter. I forgot that this was shared with the other instruments.

JRehling

Oct 10 2005, 03:07 PM

QUOTE (jmknapp @ Oct 9 2005, 01:46 PM)

How bright is Saturn-shine, compared to the normal sunshine?

How good of a picture can they get of the shadowed side under a full Saturn?

It depends on which moon, and which phase Saturn is in from the moon's perspective. To answer this question without doing any math, I can tell you that they got very good saturnshine images of Iapetus, so have no fears about Dione, which receives orders of magnitude more saturnshine. The downside is that closer moons mean higher relative velocity, and thus shorter exposure times, but the added luminance will surely make up for that. We should get great images.

Decepticon

Oct 10 2005, 03:09 PM

I really want the scan platform now.

Out of all the things to cut from the final design.

ugordan

Oct 10 2005, 03:23 PM

QUOTE (Decepticon @ Oct 10 2005, 05:09 PM)

I really want the scan platform now.

Out of all the things to cut from the final design.

Look at the bright side, instrument pointing turned out to be more accurate and steady (Dr. Carolyn Porco described the spacecraft as a "tripod in space" ) this way than it would have been with a scan platform. I don't miss the scan platform that much though it would have been nice to have a real-time downlink capability on Cassini instead of the 15 hr observation/9 hr downlink scheme...

So, once again, even the tiniest moons of Saturn have the ability to throw totally unexpected curves at us.

As for Dione itself, it's turned out to be rather strange itself -- that lacework of tectonic cracks across its surface goes down to the very finest scales imaginable, which one doesn't seem to see on Tethys (or at least on the parts of it we've viewed). And they're very often perfectly straight!:

But then why isn't the same thing happening to Tethys' comparably small craters -- and there's no sign at all that it's happening there?:

Telesto is at the L4 point, perhaps that acts as a trap gathering more material building up into a fluffy layer. Later impacts could be levitating this layer causing it to spread, smoothing out the surface and burying previous impacts. Tethys' size may prevent it from accumulating a deep fluffy layer, both because it has a much larger surface area to cover and because its gravity accelerates material causing it to impact faster compressing it instead.

JRehling

Oct 13 2005, 03:34 PM

QUOTE (alan @ Oct 13 2005, 08:26 AM)

Telesto is at the L4 point, perhaps that acts as a trap gathering more material building up into a fluffy layer. Later impacts could be levitating this layer causing it to spread, smoothing out the surface and burying previous impacts. Tethys' size may prevent it from accumulating a deep fluffy layer, both because it has a much larger surface area to cover and because its gravity accelerates material causing it to impact faster compressing it instead.

I wonder what the dynamics are of rendezvous/collision at L4. It could be that the stuff coming to Telesto is coming in too slowly to vaporize water, while the stuff coming to Tethys is coming in above that threshold, giving Tethys a lunar-style impact+regolith surface and Telesto something quite different.

tfisher

Oct 14 2005, 05:12 AM

I tried producing a color image of Telesto from the raw shots that were taken, but because of movement and rotation things wouldn't line up perfectly and I couldn't figure out what the color balance should be... So, I pushed the saturation all the way up. Some of the strongest color variation comes from misalignment, some from the moon rotating in the sunlight, and maybe some comes from variations in the surface albedo.Anyway, you can see topography pretty well in this psychedelic composition: Click to view attachment

So, once again, even the tiniest moons of Saturn have the ability to throw totally unexpected curves at us.

As for Dione itself, it's turned out to be rather strange itself -- that lacework of tectonic cracks across its surface goes down to the very finest scales imaginable, which one doesn't seem to see on Tethys (or at least on the parts of it we've viewed). And they're very often perfectly straight!:

Could be. After all, somebody is obviously farming on Ganymede. (God, I hope Hoagland doesn't hear about this thread.)

edstrick

Oct 15 2005, 06:39 AM

The smooth areas on Telesto are so smooth, it's hard to see details in them, so I ran the bandpass filtering enhancement on one of the best pics.

Ignore the @#$@# JPG artifacts and look at the CREASES in the smooth fill in the (presumed) craters on the left part of the image, and zig-zagging a bit across the lowest parts of the smooth "plains" in the central part of Telesto.

Certainly, there are active processes transporting fine grained (I presume) material of the plains and obliterating most small craters quickly relative to their production rate. But what's making the creases?... GO FIGURE. I suspect drainage into a "rubble pile" interior may play a role, but why make what we see?

Thanks for the tweaked Telesto, Ed. I've been captivated by that image, too and noticed those "creases" the first time I saw this image.

My initial impression? Frost polygons. Especially the five-sided one in the center. Don't know how likely that is, but that is what it looks like.

My take on Telesto? It looks like a snow-covered rubble pile. By chance or design, some rubble has parked at the L4 point, and there is a source of fine material (water ice?) that drifts onto this rubble pile. I don't know how the observed facts correlate with this idea.

Bill Harris:"My take on Telesto? It looks like a snow-covered rubble pile. By chance or design, some rubble has parked at the L4 point, and there is a source of fine material (water ice?) that drifts onto this rubble pile."

That's more or less my take on it. Post Voyager models suggested all the mid and smaller inner sats except maybe Rhea had a significant chance of being impact disrupted and re-accreted long after the formation of the Saturn system, and it's was plausible at the time that the lagrangian moons and the like were fragments that escaped capture. I don't know the current status of the idea, though.

SigurRosFan

Oct 16 2005, 10:36 AM

Decepticon: It's Melanthius at the pole.

helvick

Oct 16 2005, 11:00 AM

QUOTE (edstrick @ Oct 16 2005, 06:41 AM)

Bill Harris:".. or design, some rubble has parked at the L4 point..."

"Yo Zeus, where'd you put all the ice dude?"

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